Oversampling conversion techniques allow for a reduction in the amount of analog circuitry required in analog-to-digital and digital-to-analog converters by shifting most of the digital processing functions into the digital domain. Despite the use of oversampling conversion techniques, however, the need for analog circuitry is not completely eliminated. Those analog elements still necessary for the implementation of oversampling conversion, such as operational amplifiers, must meet stringent performance requirements, especially in light of the high sampling frequency required. In addition to meeting the stringent performance requirements necessary for oversampling conversion, these analog elements must be compatible with the technology used to fabricate the associated digital integrated circuits such that both the analog and digital functions can be integrated into a single high-density chip. For example, if complementary metal oxide semiconductor (CMOS) digital circuitry is being used, the associated analog circuit elements must be fabricated using CMOS technology or CMOS compatible technology.
Complementary metal oxide semiconductor (CMOS) oversampled converters typically include continuous parts, such as RC anti-aliasing and smoothing filters, directly connected to switched capacitors being sampled at high-frequency, typically in the range of 1-10 MHz. Currently available CMOS converters implement CMOS operational amplifiers for switched capacitor filters which are generally based on single-stage transconductance amplifier structures. While single-stage transconductance operational amplifier structures have speed characteristics suitable for oversampled converters, these structures present a very high output impedance such that they are not well adapted to driving resistive loads. Further, the output stages of currently available CMOS amplifiers typically consist of several transistors connected in series, a structure which limits the available output voltage swing. A consequence of reduced output voltage swing is decreased output linearity and signal-to-noise ratio performance. Finally, available amplifiers generally using continuous RC filters are not suited for use with high-frequency sampled switched capacitor circuits.
Thus, a need has arisen for a CMOS operational amplifier adapted to the performance required for oversampled converters. Specifically, the need has arisen for a high-speed operational amplifier having high output linearity, improved signal to noise ratio performance at the output, and having a reduced output impedance capable of driving resistive loads.